There are several ways to categorize ectopic tissues. We have created a classification for each system to provide better understanding. Our classification is based on location. In addition, key points of pathogenesis and basic knowledge of the embryogenesis of ectopic tissues are summarized. (Figure-1)

Brain

 1-Ectopic brain tissue

Periventricular heterotopia & Subcortical heterotopia

Disruption of cerebral cortex development can lead to abnormalities like neuronal heterotopias, which involve neurogenesis, migration, and development. Heterotopia arises when neurons miss their expected locations, classified as periventricular, subcortical, or leptomeningeal. Periventricular heterotopia (PNH), the most common subtype. Subcortical heterotopia refers to gray matter located in the white matter between the cortex and lateral ventricles (Figure-2,3,4)

Nasal glial heterotopia

Nasal glial heterotopia (NGH) is a rare congenital, non-neoplastic displacement of cerebral tissue in extracranial sites.

Preoperative imaging is vital for locating the lesion and assessing its size and extent (Figure-5). 

2-Ectopic posterior pituitary gland

The pituitary gland consists of an anterior lobe (adenohypophysis) and a posterior lobe (neurohypophysis).  The neurohypophysis forms from a downward extension of diencephalon tissue from the third ventricle, surrounding the adenohypophysis. The pituitary stalk links the hypothalamus's median eminence to the gland.

Ectopic posterior pituitary (EPP) is a rare disorder defined by abnormal placement of the posterior pituitary lobe (Figure-6,7) and agenesis of the infundibular stalk (AIS). Its cause is unclear, but some genetic mutations have been linked to EPP, which may occur alone or with other conditions.

 Head and Neck

 1-Ectopic thyroid tissue

An ectopic thyroid can be described as functional thyroid tissue located somewhere other than its anatomical position, which can occur via the migratory route, mainly along the thyroglossal duct. But it can rarely be found in the mediastinum, heart, diaphragm, and esophagus. The thyroglossal duct is a channel-like structure that forms a path from the foramen cecum to the thyroid cartilage. The most common subtype is the lingua, particularly the base of the tongue (approximately 90% of cases) (Figure 8-9).

2-Ectopic parathyroid gland

Humans usually have four paired parathyroid glands: two superior and two inferior, though this can vary. The superior glands originate from the fourth pharyngeal pouch and settle near the cricothyroid junction, behind the thyroid and recurrent laryngeal nerve. The inferior glands come from the third pharyngeal pouch, descending with the thymus, and are typically located near the inferior thyroid, in front of the recurrent laryngeal nerve. Their location can vary along the descent pathway of the branchial pouches (Figure-10).

Ectopic parathyroid glands arise from developmental migration issues. Superior glands are near the tracheoesophageal groove or retroesophageal region, while inferior glands are in the anterior mediastinum near the thymus or thyroid. These glands are linked to hyperparathyroidism.

3-Ectopic cervical thymus:

The thymus, a crucial lymphoid organ, is vital for the immune system. It forms during the sixth gestational week from the third and fourth pharyngeal pouches. By the seventh week, thymic buds elongate into the thymopharyngeal duct and migrate to the superior mediastinum.  Ectopic tissue may arise from incomplete duct migration, commonly in the cervical region as ectopic cervical thymus (ECT) (Figure-11,12,13). It is usually found incidentally but can cause compressive symptoms, often presenting as a painless mass. Diagnosis is challenging due to similarities with other lymphoid tissues, making histopathological examination the only confirmatory method.

Thorax

 1-Ectopic Lung Tissue

Ectopic lung tissue is a rare developmental abnormality of unknown origin, likely arising from the tracheobronchial tree during the third week of gestation and typically diagnosed through histopathologic evaluation.

2-Ectopic Breast Tissue

During embryogenesis, the galactic band extends from the axillae to the groins, forming breast tissue in the pectoral region. If regression fails, ectopic breast tissue develops along the milk line, chiefly in the axilla (60-70%). This ectopic tissue experiences physiological and pathological changes like eutopic breast tissue, including lactational changes and benign or malignant conditions. The most common issues with accessory breast tissue are cancer, followed by mastopathy and fibroadenoma. Surgical treatment is done for cosmetic reasons or discomfort.

Gastrointestinal tract

1-Heterotopic gastric mucosa:

Heterotopic gastric mucosa (HGM) is gastric tissue located outside the stomach, often found in the esophagus, Meckel diverticulum, and enteric duplication cysts, though it can occur anywhere in the gastrointestinal tract. Its symptoms vary by location and size, potentially causing gastrointestinal bleeding. Diagnosing HGM preoperatively is challenging; however, CT scans, fluoroscopy, capsule endoscopy, and 99mTc pertechnetate scans can be utilized, with specific features evident in Meckel's diverticulum. Scintigraphy is the preferred diagnostic method for HGM in Meckel diverticulum.

2-Ectopic pancreas:

Ectopic pancreas has variable naming as accessory or heterotopic. The main definition is the anatomically separate pancreatic tissue from the main gland without vascular and ductal continuity. The most common location for heterotopic pancreas is the upper gastrointestinal tract. It occurs in the submucosa of the gastric antrum (30%) and proximal portion of the duodenum (30%) in close proximity to the normal pancreas (Figure-14,15). This entity is generally asymptomatic and is discovered incidentally in unrelated illnesses. Symptoms depend on the location and association with or without complications. Epigastric pain is the most common symptom, followed by chronic abdominal pain, nausea, vomiting, and rarely, weight loss.

3-Ectopic Spleen

This review highlights wandering spleen, a rare condition due to ligament laxity. Symptoms vary based on the ectopic site (abdomen or pelvis) or complications like torsion (Figure-16,17,18,19).

Genitourinary tract

1-Adrenal rest tissue

Ectopic adrenal rest tissue is adrenal tissue located outside the adrenal glands, typically near the celiac plexus, retroperitoneum, or along the gonadal descent, most often in a testis or spermatic cord cord.

The adrenal gland consists of two parts: cortex and medulla, arising from the intermediate mesoderm between the genital ridge and neural crest near the dorsal aorta. During embryogenesis, the cortex is close to the gonads, suggesting adrenal rests in males descend with the testes and contain only cortex, while those near the gland may include medulla. This is commonly linked to testicular adrenal rest tumors. Patients usually show no symptoms, but in congenital adrenal hyperplasia (CAH), high adrenocorticotropic hormone levels activate adrenal rest tissue (Figure-20,21,22,23).

2-Ectopic kidney

Kidneys develop through three fetal stages: pronephros, mesonephros, and metanephros. Development starts in week three with pronephros, followed by mesonephros at four weeks and metanephros at five weeks. The metanephros begins in front of the sacral somites, migrating and rotating 90° to its final pelvic position. Renal ectopia, caused by faulty migration along the cranial-caudal axis, commonly results in pelvic kidney positions, although iliac, abdominal, and thoracic positions can occur (Figure-24). Ectopic kidneys are often smaller and irregularly shaped due to fetal lobulations, with atypical vascular supply usually from the iliac artery or aorta. Coexisting urogenital abnormalities occur in 15% to 45% of cases, including contralateral renal anomalies, hypospadias, cryptorchidism, and uterine or vaginal anomalies in females.

3-Adenomyosis & Endometriosis

Adenomyosis features ectopic endometrial tissue in the myometrium, often seen as endometriosis, causing muscle cell hypertrophy and hyperplasia. Diagnosis is difficult due to non-specific symptoms like chronic pelvic pain, dysmenorrhea, menorrhagia, and abnormal uterine bleeding, frequently with other hormone-related disorders. MRI is preferred for differentiating diffuse and focal adenomyosis (Figure-25,26,27).

Endometriosis is a chronic condition with endometrial-like tissue growing outside the uterus, similar to adenomyosis. The types include superficial endometriosis, endometriomas, and deep infiltrating endometriosis. Laparoscopy is the gold standard for diagnosis. It often causes pelvic pain, infertility, and unusual symptoms that delay diagnosis (Figure-25,28,29).